Intelligent Optimization Design of a Deepwater Semi-Submersible Platform Mooring System Based on Multi-Task Shared Neural Networks

As the offshore industry looks beyond water depths of 3,000 ft for future oil and gas production, the industry standard steel catenary spread mooring (CSM) system used on all floating production systems installed to date becomes increasingly inefficient and costly, An alternative to the CSM is the Taut Leg Spread Mooring (TLSM) system with its characteristic short scope legs and vertically loaded anchors. In water depths greater than 3,000 ft, it has been shown that TLSMS comprised of synthetic mooring lines have performance advantages over systems utilizing steel wire rope and demonstrated potential for significant cost reductions. Early studies investigated the use of aramid fiber rope due to its high strength, lower in-water weight and lower axial stiffness as compared to ropes made from steel. Later studies indicated that the material properties of polyester fiber, primarily it's lower Young's modulus, made polyester mooring lines more suitable for use in TLSMS2'S. As the TLSM knowledge base expanded, aramid rope construction evolved. Earlier efforts to match the characteristics of steel wire rope have given way to new generation 'soft' aramid constructions. This paper compares the mooring performance and total installed cost of a "soft? aram id TLSM to a typical polyester configuration. Both systems were designed for use in the Gulf of Mexico (GOM) with an FPS based on the Aker P45. Introduction This study was performed with the objective of investigating and comparing two taut leg spread mooring systems comprised of differing types of synthetic rope. The two types of synthetic mooring line constructions investigated were:Whitehill 36-strand aramid rope.Marlow Superline parallel-strand polyester rope. Mooring systems using both constructions were designed for the new-build Aker F'45 Semi-submersible Drilling and Production platform (FDPU), The first P45 is currently being constructed for Norsk Hydro for their Njord field development. The vessel's displacement will be 49,600 tons when completed; production facilities will be capable of 70,000 BOPD of crude throughput. The particulars of the Aker P45 are presented in the Appendix to this document. The study this paper is based upon has been planned and organized such that it would be a suitable extension to work previously presented13 A deep water (3,000 ft) GOM location was chosen as the field location for this work. Three environmental design scenarios were investigated for this site. They are:Maximum Wave Hurricane Event with associated wind and currentMaximum Loop Current Event with associated wave and windMaximum Wave Case -5 year return The Maximum Wave Hurricane condition noted above was the controlling case and, therefore, was used in the design effort. The goal of this study is to evaluate whether mooring lines constructed of aramid fiber are cost and performance competitive with polyester for deep water taut leg spread moorings. As part of the scope of the study, installation costs and schedules have been developed for each system. Preliminary component specifications were developed to a level of detail where costs could be estimated for each mooring case.

The evaluation of existing mooring system and the development of an appropriate optimization design became the key issues in the Liuhua 11–1 FPS dry-dock updating project because this optimized mooring system has to face new challenges, such as the update of site metocean conditions, the revision of wind force coefficients due to upgraded topsides, and the additional corrosion allowance for chain links due to extra 15-year service life extension compared with the original system design. A design team was formed to give the evaluation of the existing mooring system based on mooring line tensions and offset limits according to the latest industry standard for a permanent mooring system. Since the existing mooring system marginally failed the design requirements based on the evaluation, system optimization design focused on the adjustment of mooring line pretensions has been carried out to improve the mooring system performance and eventually to satisfy the classification requirements for the tension safety factors and the platform offset limits driven by the riser designs. The mooring system analysis was conducted in time domain. The Ariane program developed by BV was used in the detailed mooring system analyses. For the calibration and verification purpose, the system evaluation and the optimization design had also been independently verified with DNV's DeepC program. Introduction The LH 11–1 oil field is located about 200km southeast of Hong Kong in the South China Sea in approximately 310m depth of water. LH 11–1 FPS, which named ‘Nanhai TiaoZhan’, is a major component of LH 11–1 oil field development. The FPS was a modified drilling semi-submersible converted from a Sedco 700 series drilling rig built in 1975 and in 1995, it was converted again to a floating production unit in Singapore and then deployed at LH 11–1 oil field, permanently moored by 11 mooring legs. The Unit is classed by American Bureau of Shipping (ABS). It was originally designed for a service life of 10 years (from 1995) and has been continuously operating for more than 15 years. At the same time, a new oil field Liuhua 4–1 (OTC-24042), about 11 km away northwest LH 11–1, was in development planning with the utilization of LH 11–1 FPS facilities through a subsea tieback. Therefore, additional topsides capacities and the life extension of this aged semi-submersible FPS became necessary. CNOOC Ltd, Shenzhen is the owner of this LH 11–1 FPS. COOEC was selected as the contractor for the engineering and construction support of this FPS dry-docking upgrading and life extension project. CSSC Guanzhou Huangpu Shipbuilding Co., Ltd was contracted by the owner as the dry-docking shipyard for the upgrading construction.

This paper discusses key technical issues for classing deepwater mooring systems, Special applications of new mooring components such as synthetic fiber ropes, suction pile anchors and Vertically Loaded Anchors (VLA) have been presented. This paper establishes the design criteria for synthetic fiber ropes, suction pile anchors, and VLAs. The rationale as presented is based on recent projects and research and development results from joint industry studies. Testing and installation considerations for synthetic fiber ropes and field test requirements for suction pile anchors and VLAs are reviewed and discussed. Additional design and analysis requirements for the use of synthetic fiber ropes are also proposed for a taut leg mooring system. This paper represents a summary of experiences gained through the application of these innovative mooring technologies in mooring components and anchoring devices. The guidelines set by ABS as presented in this paper are intended to enhance safety standards in marine operations while meeting the new challenge to moor floating units in deep water. Introduction With the recent explosive growth in deepwater drilling and production operations worldwide. American Bureau of Shipping (ABS) is actively involved in classification of several conversions to deepwater and new-built floating platforms including semi-submersibles, ship-shaped drilling units and tanker-based FPSOs (Floating Production. Storage and Oftloading), Most of these floating units will be moored in the ever increasing water depths. In deep water, the conventional catenary mooring systems become less stiff and have a larger horizontal spread, with much higher mooring component costs and payload penalties. To cope with these challenges, new mooring systems for stationkeeping of floating units have been developed for ongoing projects. such as those offshore of Brazil installed or to be installed. by Petrobras in 1997 and 1998. These include polyester synthetic fiber ropes. VLA and suction pile anchors applied in the taut leg mooring systems. Classification of Deepwater Mooring Systems General. ABS has established the position that the scope of classification of an FPSO system shall include three major items, Le., the mooring system, the vessel, and the production facilities [Ref. 1]. Other equipment and systems such as import and/or export risers may also be classed, if requested by the owner. Since the early 1970s, ABS has provided numerous classification services for floating production systems. Table I lists some recent projects in water depths greater than 500 meters. There are special considerations to be addressed for the design and analysis of mooring systems in deepwater applications. The key element, when it comes to novel systems involving new components such as synthetic fiber ropes, suction anchors, vertically loaded anchors, etc., is to provide a comparable and compatible safety standt1rd to that of the traditional catenary mooring with chain wire and drag embedment anchors. It is well recognized that the mooring line loads tend to favorably adjust themselves when movements of drag embedded anchors occur in the traditional catenary mooring system, This is the rationale for the safety factors of the anchor holding capacity being much lower than those of the mooring line tension,

This paper presents model test and analytical correlation results of a single point moored Floating Production, Storage and Offloading tanker based system (FPSO). Both Permanent and Disconnectable turret mooring system are considered. Analytical calculation of green water effects, vessel motions, turret and mooring line loads are compared with model test results. Frequency and time domain analysis techniques of calculating line tensions are demonstrated and results are compared to the model tests measured time histories. The analysis procedure discussed in this paper is aimed toward estimating I)motions at bow relative to the water surface, 2)mooring line tensions and 3)turret design loads for a turret moored FPSO with a reasonable degree of accuracy. it is demonstrated that mooring line tensions can be calculated accurately with a finite element time domain computer simulation of the line dynamics provided the input fairlead motions are correct. Thus the method of computing time histories of mooring line tensions based on free-floating vessel motions is applicable for other spread moored floating systems in the same range of water depth. CONCLUSIONS AND RECOMMENDATIONS The model test results confirm the feasibility of both types of turret moored systems and validate the analytical tools and procedures. Results derived from the time domain line dynamic analysis agree well with the model tests. The dynamics of slack line are over estimated by using the frequency domain approach. Two methods of API mooring line fatigue calculation are compared. This paper demonstrates that a frequency domain quasistatic approach provides an efficient tool for initial sizing of mooring components. The maximum global mooring line and turret loads may be estimated by combining the frequency domain line dynamic results with the quasistatic approach. A time domain line dynamic approach can confirm and/or predict the maximum line tension with a high level of confidence, provided the vessel motion at the line top can be prescribed accurately. INTRODUCTION A turret moored 140,000 DWT tanker system is being designed for 1,000 feet of water in the South China Sea. The turret moored tanker is an attractive concept for both production facilities and offshore storage. Design of the weather vaning turret mooring system and riser system connected to the seafloor are two key system components for FPSO development. In the design of a turret mooring system the prediction of the extreme motion responses of the moored vessel is of the utmost importance. A moored tanker exposed to severe environmental conditions experiences motion responses in two distinct frequency ranges:Wave frequency responses around the wave period range (12-15 seconds).Low frequency responses around the tanker natural period range (100 seconds); The wave frequency response is generally well predictable, whereas estimates on the low frequency responses are more difficult and may vary by an order of magnitude depending on analytical tools and design procedures. Often a well executed model test program in parallel with analytical approach is imperative to ensure a reliable design of the turret mooring system.

Preliminary design and dynamic analysis of constant tension mooring system on a 15 MW semi-submersible wind turbine for extreme conditions in shallow water

The design and analysis of a quayside mooring system for safe mooring of Prelude FLNG under extreme environmental conditions were carried out. The design of the mooring system considered the yard operation conditions and maximum wind speed during a typhoon. In order to secure the mooring safety of Prelude FLNG under an extreme environment, a special steel structure was designed between the quay and Prelude FLNG to maintain the distance from the quay to a certain extent to avoid a collision with the inclined base. The mooring safety was also ensured by installing additional new parts on the quay. A mooring analysis and mooring safety review were performed with more rigorous modeling considering the nonlinearity of the mooring rope and fender. In order to secure additional safety of the mooring system under extreme environmental conditions, a safety assessment was conducted on the failures of the mooring components proposed in the marine mooring guidelines. Based on the results of the mooring analysis, it was confirmed that the Prelude FLNG can be safely moored even under the extreme conditions of typhoons, and a worst case scenario analysis verified that the mooring system design was robust enough. The proposed mooring analysis and design method will provide a basis for the safe mooring of ultra-large floating offshore structures of similar size in the future.

Mooring line snap loads occurs more frequently for floating offshore wind turbines (FOWTs) deployed in intermediate water depths (50–100 m) due to the use of shorter mooring lines compared to deep water applications (>100 m). Current design standards estimate maximum mooring line tension values based on the assumption that snap load occurrences are rare. However, this approach risks underestimating the maximum design tensions in systems where snap loads occur frequently. In this study, different mooring systems for a 15MW VolturnUS-S reference FOWT targeted for deep (200 m) and intermediate (60 m) water depths are investigated. The analysis focuses on the effect of water depth on the occurrence of snap loads and their influence on maximum mooring line tensions. The chain-catenary mooring (Cat-200) system, designed for 200 m water depths, serves as the baseline for comparison against two alternative mooring systems for intermediate waters, i.e. the chain-catenary (Cat-60) and taut (Taut-60) systems. Dynamic analysis results show that the mooring systems in the intermediate water depth result in higher tension standard deviations compared to that of the deep water mooring system, due to their stronger wave-frequency responses. Moreover, the maximum mooring line tensions under the 50-year storm environmental condition are analysed using the mean up-crossing rate (MUR) method with a threshold level determined experimentally. Among the systems analyzed, the Cat-60 mooring system experiences the highest maximum mooring line tension—348% and 95% greater than the Cat-200 and Taut-60 systems, respectively. In terms of the snap load occurrence, the Taut-60 mooring system is effective in reducing the occurrence of snap events, offering a more stable performance. Lastly, results in this study also indicate that the MUR method accurately predicts maximum mooring line tensions for systems with both high and low snap event occurrences.

Deep-sea aquaculture can alleviate the spatial and environmental pressure of near-shore aquaculture and produce higher quality aquatic products, which is the main development direction of global aquaculture. The coastline of China is relatively flat, with aquaculture operations typically operating in sea areas with water depths of approximately 30–50 m. However, with frequent typhoons and poor sea conditions, the design of mooring system has always been a difficult problem. This paper investigated the multiple cages, considering two layouts of 1 × 4 and 2 × 2, and proposed three different mooring system design schemes. The mooring line tension of the mooring systems under the self-storage condition was compared, and it was observed whether the mooring line accumulation and the contact between the mooring line and the steel structure occurred on the leeward side. Additionally, flexible net models were compared with rigid net models to evaluate the impact of net deformation on cage movement and mooring line tension. Finally, based on the optimal mooring design, the dynamic response of the mooring system under irregular wave conditions was analyzed and studied, providing important reference for the safety and economic design of the mooring system of multiple fish cages.

Enclosure aquaculture is a healthy and ecological aquaculture pattern developed in recent years to relieve the pressure due to the wild fish stock decline and water pollution. The object of this paper was a floating rope enclosure, which mainly consisted of floaters, mooring lines, sinkers and a net. In order to optimize mooring design factors, the hydrodynamic responses of the floating rope enclosure with different mooring systems in combined wave-current were investigated by experimental and numerical methods. Physical model experiments with a model scale of 1:50 were performed to investigate the hydrodynamic characteristics of a floating rope enclosure with 12 mooring lines. Based on the lumped mass method, the numerical model was established to investigate the effects of mooring design factors on the mooring line tension, force acting on the bottom, and the volume retention of the floating rope enclosure. Through the analysis of numerical and experimental results, it was found that the maximum mooring line tension of the floating rope enclosure occurs on both sides of the windward. Increasing the number of mooring lines on the windward side is helpful to reduce the maximum mooring line tension. Waves and current both have an influence on the mooring line tension; in contrast, currents have a more obvious effect on the mooring line tension than waves. However, the influence of the wave period on the maximum mooring line tension is small. The force endured by the bottom of the floating rope enclosure also changes periodically with the wave period. Yet, the maximum force endured by the bottom of floating rope enclosure occurred at the windward and leeward of the structure. The volume retention of the floating rope enclosure increased with the increasing amount of mooring lines.

To investigate the hydrodynamic capabilities of the floating breakwater with a taut mooring system, a two-dimensional physical modelling experiment was performed to investigate the hydrodynamic characteristics of the hydrodynamic pressure on the front surface of the floating box, mooring line tension and transmission coefficient under regular waves, considering factors such as mooring angle, mooring line pre-tension and relative width. The results show that, with the increase of the mooring angle, the hydrodynamic pressure on the floating box first increases and then decreases, the mooring line tension first increases and then decreases slightly, and the transmission coefficient obviously decreases. As the mooring line pre-tension increases, the hydrodynamic pressure and the mooring line tension increase, and the transmission coefficient first decreases and then increases. As the relative width increases, the hydrodynamic pressure, mooring line tension and transmission coefficient all show a decreasing trend. The results can provide a reference for the design and safety assessment of taut moored single pontoon floating breakwater.

This paper presents a probabilistic study of various Gulf of Mexico (GoM) MODU mooring systems that was performed as part of the MODU Mooring JIP (Joint Industry Project). The intent was to determine the relative changes in reliability between different types of MODUs, with different mooring systems, in different water depths, designed for different hurricane conditions and located at different regions in the Gulf. In addition, the effects of assuming reduced mooring line strength were investigated. Twenty cases were selected which cover four different classes of MODUs, four different types of mooring systems, three different numbers of mooring lines, as well as three different water depths. These base cases were chosen so that trends or differences between different hull shapes and mooring systems could be identified. These twenty base cases are further expanded into sixty cases according to location in the Gulf as well as the return period used to design the mooring system. The results indicate that on average the " notional?? (i.e., relative) annual failure probability of mooring line failure decreases by one order of magnitude when increasing the MODU mooring system design return period from 10-yr to 50-yr. Two orders of magnitude decrease are observed if the design return period is increased from 10-yr to 100-yr. In addition to the above findings, reliability sensitivity to four different GoM Metocean regions is also provided. Redundancy of the mooring system for 8, 12 and 16-lines is also examined in terms of the additional reliability gained by the extra lines. Introduction The probabilistic reliability approach was performed using an updated procedure that was originally developed in prior work by DeepStar [1]. The results can be used by operators, drilling contractors, API work groups and regulators alike to understand, on a comparative basis, the performance of various MODUs and mooring systems. A base set of 20 separate MODU and associated mooring cases were evaluated as shown in the first twenty cases of the matrix in Table 1. These selected twenty base cases cover four MODUs (or MODU classes), four types of mooring systems, three numbers of mooring lines and three water depths. The footnotes of Table 1 provide definitions of these cases. These base cases were chosen so that trends or differences between different hull shapes and mooring systems can be identified. This base set of cases was then further defined into a larger set of 60 according to location in the Gulf as well as the return period used to design the mooring system. The Gulf locations were defined as the Central, West Central and West according to MODU JIP metocean study [5] and the latest API guideline [6], and the design return periods were defined as 10, 25, 50 and 100 year. The remainder of Table 1 shows the resulting matrix of cases studied. The full set of 60 cases was initially evaluated for reliability using 100% CBS (Catalogue Break Strength) for the mooring line. The entire set of 60 was then re-evaluated using 80% CBS to determine the sensitivity of the results to lower than expected mooring line strength. For the 8-10 mooring line cases, the first and second line failures were determined. The second line failure was used to represent the mooring system failure. For the 12 and 16 line cases, the third line failure was also determined, with the third line failure representing the mooring system failure. This allowed a comparison of the effect of mooring line redundancy in terms of the first line failure, versus the mooring system failure.

With the global oil and gas resource development accelerating its expansion into deepwater and ultra-deepwater areas, taut mooring systems have gradually become key technical equipment. However, the complex coupling mechanism between the material parameters of polyester cables and their dynamic responses has not been fully revealed, and existing design methods mostly rely on simplified models, lacking systematic verification with real environmental data and dynamic responses. Based on on-site platform data, this study proposes a multi-dimensional integrated collaborative optimization design scheme for deepwater tension leg mooring systems, aiming to provide guidance for mooring design projects in similar sea areas and enhance the safety and economy of deepwater mooring systems. To address the nonlinear optimization challenges of multi-body coupled systems, this study establishes a time-domain multi-body coupled model of the platform-riser-taut mooring system and conducts systematic research in combination with the NSGA-II multi-objective genetic algorithm. First, a multi-body coupled time-domain simulation model is constructed based on the actual geometric dimensions of the platform structure and large-scale measured environmental data, covering wave-frequency and low-frequency dynamic response characteristics. Second, statistically representative working conditions are adopted to verify the accuracy of the model's dynamic response. Third, a global sensitivity analysis is conducted to quantify the influence weights of polyester cable parameters and reveal the main control factors of the mooring cable's dynamic response. Fourth, with API-RP-2SM as the constraint, a multi-objective optimization function of the standard deviation of mooring tension and the extreme value of platform displacement is established to solve the Pareto frontier of parameter combinations. Finally, the optimization effect is verified based on typical working conditions, forming a full-process analysis method for mooring optimization. This study provides a digital solution with practical engineering value for the design of deepwater mooring systems. The research results show that: (1) The environmental loads in the target sea area exhibit significant quarterly periodic characteristics, with extreme working conditions in the first quarter accounting for 1%. (2) The length and diameter of the polyester cable have a significant impact on mooring tension. When the length of the polyester cable increases from 1850m to 2000m, a single variable of 2.5% causes the mooring tension to change by approximately 60%. The innovation of this paper lies in proposing an optimization scheme for the mooring system based on the real data from the platform. This method combines environmental data with multi-objective optimization algorithms, breaking through the limitations of traditional empirical design. At the same time, based on the actual data from the site, representative working conditions are selected to support model verification, revealing the quantitative influence laws of polyester cable material parameters on the non-linearity of mooring tension, so as to provide real guidance for similar engineering design and construction in similar sea areas.

The influence of mooring system in rogue wave impact on an offshore platform

As nearshore aquaculture spaces become saturated, the development of fisheries aquaculture for deep sea has become an inevitable trend. This paper proposes an innovative deep-sea aquaculture platform that incorporates a vessel-shaped main structure and a single-point mooring system. The potential flow theory and the Morison equation are utilized to calculate the hydrodynamic loads on the main structure and the netting and mooring systems, respectively. The deformation and force of the netting in current are simulated, and the accuracy of the analytical methods used is validated based on experimental results. The influences of the netting system on the hydrodynamic characteristics of the platform are analyzed. Optimization on the single-point mooring system is conducted under static and dynamic conditions, considering the influences of various mooring parameters, including mooring line length, buoyancy of buoys, and mass of sinkers. The patterns of changes in motion response, mooring line tension, and minimum touchdown length under different mooring parameters are calculated and analyzed. The results indicate that changes in mooring line length have minimal impact on the dynamic response of the platform and mooring system. The addition of appropriate buoys or sinkers can reduce the motion response of the platform and the tension in the mooring lines. Moreover, compared to adding buoys, incorporating sinkers more effectively enhances the overall safety and stability of the platform system.

The studies on floating offshore wind turbines (FOWTs) have been increasing over recent decades due to the growing interest in offshore renewable energy. The present paper proposes a barge platform with four moonpools to support the Technical University of Denmark 10 MW wind turbine for a designed water depth of 60 m. A 4 × 2 mooring system with eight mooring lines is also proposed for the barge platform. The main dimensions of the barge platform are optimally selected with respect to its preliminary hydrodynamic properties and potential financial benefit. The proposed barge-type FOWT is then demonstrated to be aligned with the DNV standard requirements in terms of its intact and damage stability. Furthermore, coupled time-domain simulations are conducted for the proposed barge FOWT with mooring under the selected environmental and operational conditions by using Simo-Riflex-AeroDyn (SRA). Through decay test simulations, the natural periods of the barge-type FOWT are demonstrated to be within the DNV recommended ranges. The proposed mooring system is also benchmarked with the 3 × 3 mooring concept that was used for a 3 MW barge-type FOWT installed in Kitakyushu. The response magnitudes of the barge platform and mooring line tension are similar to both mooring systems, and thus the 4 × 2 mooring system is preferred due to its lower cost. In addition, the proposed barge platform is preliminarily demonstrated to be able to survive for the 50-year extreme environmental conditions under parked wind turbine status, as well as the normal environmental conditions under the operating status.